Plastic tubing denotes a particular diameter schedule of plastic pipe in which the outside diameter of the tubing is equal to the nominal size plus 3.175 mm or 0.125″ (inch). Plastic pipe outside diameter schedule conforms to ANSI B 36.10. For convenience, and in deference to common usage, plastic tubing having a nominal diameter in the range from 7 mm to 152 mm is referred to hereinafter as “pipe”.
ASTM D3350 Section 2 dictates that for adequate protection against ultraviolet (or “u-v”) radiation, PE pipe contain a minimum of 2 weight percent carbon black. Consequently, commercially available PE pipe in a wide variety of specifications is made to contain at least 2% by weight (“wt.”) carbon black, typically about 2.5% by wt. Because PEX pipe is typically covered up or buried in use, protection against u-v light is not a concern unless the pipe is to be exposed or stored outdoors for an extended period; PEX is therefore not generally protected against u-v light and does not contain any carbon black. During construction of structures, PEX pipe may be exposed to u-v light for up to 3 months, and in such instances, u-v protection is desirable.
PEX is conventionally crosslinked using one of several processes such as those disclosed in U.S. Pat. No. 4,117,195; U.S. Pat. No. 5,756,023 and U.S. Pat. No. 6,284,178. These crosslinking processes include addition of peroxide, addition of AZO compounds, electron beam irradiation, and addition of silane. Such crosslinking is known to enhance certain physical and chemical properties of the polyethylene. In particular, crosslinking has been shown to increase maximum useful temperature, reduce creep, improve chemical resistance, increase abrasion resistance, improve memory characteristics, improve impact resistance, and improve environmental stress crack resistance of polyethylene materials.
Though PEX has the foregoing good properties, PCT publication WO 99/49254 teaches protecting it exteriorly with an oxygen barrier layer; and U.S. Pat. No. 4,614,208 discloses a multilayer PEX pipe having an intermediate layer of oxygen diffusion resistance material and an outer layer of impact resistant polyethylene; the specific problem of protecting PEX pipe against attack by chlorine and HOCl in potable water has, to date, been ignored.
It is well recognized that PEX needs to be protected from oxidative degradation, but it is also well known that chlorine and hypochlorous acid (HOCl) are just as detrimental to PEX pipe as oxidizing agents such as oxygen, sulfur dioxide and oxides of nitrogen in the atmosphere, if not more so. Usually, antioxidants are added to protect the PEX materials from oxygen entering the material from the atmosphere. When PEX is used for piping in water distribution systems, oxidative degradation occurs not only from the oxygen in the atmosphere but also from oxidizing agents migrating from water inside the pipe. Oxidizing agents in the water, such as chlorine and HOCl are more detrimental to the PEX pipe than oxidizing agents in the atmosphere and such antioxidants as are used to protect the PEX pipe are consumed quickly when subjected to strong oxidizing agents such as chlorine and HOCl; in particular, the effectiveness of the antioxidants is diminished when the PEX pipe is exposed to u-v light. Moreover, large amounts of antioxidants cannot be used due to strict potable water extraction standards.
If one was to recognize the importance of protection not only from outside the pipe but also from within, it is evident that an effective solution would be to have a more effective antioxidant or antidegradant incorporated in the PEX. But, there is no suggestion in the art what material or “dopant” would provide such properties. Further, there is no suggestion that, should such dopant be used, and its effectiveness is to be enhanced, such enhancement might be obtained with an inner tubular core of oxidation-resistant material having a wall of thin cross-section in the range from as thin as 0.025 mm (1 mil) for 7 mm (0.25″) nominal diameter pipe, to 1.52 mm (0.06″) thick for 152 mm (6″) nominal diameter pipe. Nor is there any suggestion that such inner core be cohesively bondable to carbon-doped PEX, and co-extrudable in such a thin cross-section under substantially the same extrusion conditions as carbon-doped PEX.
From the foregoing, it will be evident that the problem of coping with degradation of PEX pipe is addressed in diverse ways, few of which are focused on the detrimental long term effects of chlorine deliberately added to water. The effectiveness of carbon black as a dopant in PEX pipe no greater than 152 mm in nominal diameter, optionally reinforced with an inner tubular of thin-walled core of high density polyethylene (“HDPE”) or chlorinated PE is judged by its resistance to degradation by chlorine and HOCl in water at elevated temperature above about 80° C., and elevated pressure above about 274 kPa (25 psig) over a long period of time more than 20 years.